Automatic frequency control (AFC) is provided by this invention for varactor tuning systems such as the one described in copending application Ser. No. 684,339 filed May 7, 1976, now abandoned, in the name of Arthur N. Borg and Daniel A. Heuer, common assignee, hereby incorporated by reference. Specific circuitry for developing a control signal for keeping a digital varactor tuning system tuned to a given signal is provided.
Varactor tuners are well known in the art. In recent times digital techniques have been used to provide tuning voltages for tuning the varactor tuner. These systems all have in common the generation of a multibit binary word for identifying a particular tuning voltage to be applied to the varactor tuner. As with other tuning systems, it is desirable to provide a correction signal for maintaining the varactor tuner tuned to a received signal. The associated frequency drift between the tuning system and the received signal is continuously monitored with a frequency discriminator and the tuning voltage corrected to compensate for the frequency drift.
One technique is to convert the stored binary word into an analog voltage by means of a D/A converter. The analog voltage is then summed with the AFC voltage to provide the varactor control voltage. Another technique is to alter the stored binary word in response to the AFC voltage. This technique offers certain advantages, such as being better adapted to integrated circuit technology, and therefore more economically manufactured. This technique will also compensate for long term "secular" frequency drift which is inherent in some tuners. However, because the stored binary word is modified by this technique, there is a possibility that the stored word may be modified excessively. This can occur when the receiver is tuned to a weak signal or goes off the air.
If the station signal is weak, the discriminator which provides the AFC signal will be considerably unbalanced due to the frequency response of the receiver IF and the varactor tuning system will tune continuously for the desired tuning frequency in an effort to minimize an apparent tuning error. In the absence of a received signal (or the presence of a very weak station signal), the tuning error detected by the AFC circuit will not be diminished and the varactor tuning system will continue tuning up or down in frequency in an attempt to locate the proper tuning point. Therefore, in systems which attempt to maintain tuning by altering the binary word defining the tuning voltage it is desirable to provide additional means for disabling the AFC in the absence of a signal having sufficient strength. Requiring the received signal to have a minimum signal level before AFC action commences also allows the tuning of weak signals to a desired tuning frequency which is slightly different from the "proper" tuning point.